1. Field of the Invention
The present invention relates generally to an optical disk apparatus for optically processing information, and more particularly to an optical disk apparatus capable of optically reading/writing information at high speed.
2. Description of the Related Art
An optical disk apparatus for optically recording information on an optical information recording medium such as an optical disk or an optical-magneto disk, or optically reproducing information from the optical information recording medium has a much greater memory capacity than a magnetic disk apparatus, etc. Thus, the optical disk apparatus will be widely used as an information recording/reproducing apparatus in near future.
In the optical disk apparatus, a light beam emitted from a light source is converged on a recording surface of an optical disk through an objective lens when optical information is reproduced, a light beam having a predetermined intensity is modulated on the recording surface of the optical disk and reflected from the recording surface. The reflected beam is detected to reproduce the optical information. When information is recorded, the light beam emitted from the light source is modulated to write optical information on the optical disk. In this optical disk apparatus, the objective lens is supported movably along the optical axis for focus control, and moved in the focusing direction. In addition, the objective lens is supported movably along the surface of the optical disk in a direction perpendicular to the optical axis for information access and tracking control, and is moved in a radial direction, i.e. tracking direction. Specifically, the objective lens is kept in the focused state, and the light beam is converged on the recording surface of the optical disk so that a minimum beam spot is formed on the recording surface of the optical disk. Further, the objective lens is kept in the tracking state, thereby enabling a light beam to trace a desired track on the optical disk, e.g. a tracking guide or a tracking guide sequence. The objective lens is kept in the focused state or in the tracking state, so that the light beam is intensity-modulated, thereby recording information on the surface of the optical disk, or the information is reproduced by detecting the beam reflected from the optical disk through the objective lens.
For example, Published Unexamined Japanese Patent Application No. 63-1317 discloses a conventional optical disk apparatus, which is shown in FIGS. 1 to 4. In the optical disk apparatus, a frame 101 is provided within a housing of the optical disk apparatus. The housing is fixed by means of four leg members 102 and 103. The frame 101 extends in the Y-direction. A pair of cylindrical steel support rails 105 and 106 are distanced in the Z-direction and fixed at both ends of the frame 101. An elongated pick-up body 107 extending in the Z-directionare arranged between the support rails 105 and 106 and supported movably by the rails 105 and 106. One side end of the pick-up body 107 is provided with two steel rollers 108 which roll over the support rail 105, and the other side end of the pick-up body 107 are provided with two pairs of steel rollers 109. One pair of the two pairs of rollers 109 are distanced from the other in a direction in which the pick-up body 107 moves. The rollers 109 are supported by the other support rail 106. As seen from FIG. 4, the pick-up body 107 is supported by the support rails 105 and 106 at three support points where the rollers 108 and 109 are provided.
As shown in FIG. 1, each roller 108 provided at one side end of the pick-up body 107 is attached at a free end portion of a lever 112. The lever 112 is attached on the pick-up body via a pin 111 and is swingable in the direction of arrow R. The direction of arrow R is perpendicular to the direction in which the pick-up body moves (i.e. Y-direction and a direction opposite to Y-direction). A spring member 113 is provided to press the roller 108 on the support rail 105. Since the rollers 108 (situated at one of the three support points) are pressed on the rail 105, the other rollers 109 (situated at the other support points) are also pressed on the support rail 106 by reaction force. As shown in FIGS. 1 and 2, the frame 101 is provided with a pair of longitudinal magnetic circuits 120, each comprising a longitudinal magnet 116 and longitudinal yokes 117 and 118 engaged with one another and all extending in the direction in which the pick-up body 107 extends (Y-direction and a direction opposite thereto). As shown in FIG. 2, a pair of drive coils 121 are attached to the pick-up body 107. The coils 121 extend in parallel to the direction in which the pick-up body 107 moves. The magnetic circuits 120 generate magnetic fluxes crossing the drive coils and constitute, along with the drive coils, a linear motor for applying a moving force to a moving optical system including the pickup body 107.
However, the optical disk apparatus shown in FIGS. 1 to 4 have the following problems.
Since the rollers 108 and rollers 108 are arranged outside the drive coils 121, mass distribution increases at both end portions of the pick-up body 107. As a result, the resonance frequency of the pick-up body 107 decreases, and wide-band control operation necessary for high-speed access becomes unstable. In order to solve this problem, it is possible to set the driving center at the node of oscillation; however, in order to set the driving center at the node, it is necessary to further increase the mass at both ends of the pick-up body. Consequently, the driving acceleration speed lowers.
As shown in FIG. 3, an actuator for finely moving an objective lens 158 in a focusing direction (arrow F) and a tracking direction (arrow T) is mounted on the pick-up body 107. Thus, when the pick-up body 107 is moved at high speed for access, it is necessary to apply a force in the T-direction to cancel the acceleration of the pick-up body 107, thereby preventing the objective lens 158 from vibrating. This increases power consumption of the entire apparatus.
The pins for rotatably supporting the rollers are firmly attached on the metallic pick-up body 107. The four pins for the rollers 108 and one pin for one of the rollers 108 are firmly fixed on the pick-up body 107 and allow freedom of rotation. The other roller 108 for applying pre-load is pressed on the support rail 105. Thus, the angle of inclination of the pick-up body 107 is denifitely determined by the precision of attachment of the rollers excluding the roller 108 for pre-pressing and by the precision of attachment of the support rails 105 and 106, and there is no room for adjustment.
Since the optical disk apparatus is so constructed that the vibration occurring in the pick-up body cannot be damped between the rollers and the pick-up body 107, abnormal vibration may occur and it is difficult to achieve stable control operation.